The present invention relates to parking braking systems for vehicles having either conventional or electronically controlled braking systems.
The accepted approach to parking brakes in vehicles fitted with traditional pneumatic braking systems is to have a connection, either pneumatically, hydraulically or by cable, between a manually operated actuating device, such as a lever or valve, and some form of braking device at the individual wheels. In heavy vehicles having pneumatically operated brakes, for example, this usually involves releasing the air supply to brake actuators at the wheels to enable the brakes to be applied by strong springs whose force is normally opposed by the air supply in non parking braking modes.
This accepted approach to parking brakes in vehicles fitted with EBS uses spring brake actuators as shown diagrammatically in FIG. 1 of the accompanying drawings. In this system, a hand-operated valve 10 is used, via a relay valve 11, to allow the parking brake to be applied. The valve 10 operates on an inverse air principle in that it is arranged to release air pressure to allow the spring force of respective spring brake actuators 12 at each wheel to be applied. A suitable parking brake reservoir 14 is required to store the pressurised air for use within the system. Where the system is used with a vehicle having a trailer, a separate relay valve (not shown) is required to allow selective operation of the trailer brakes. When the drive operates the hand valve 10, an inverse pneumatic signal is produced, i.e., the pressure output from the valve 10 falls with increasing demand. This causes the spring brakes 12 to be applied since, in the normal running mode (no braking) the springs are held off by compressed air.
As evident from FIG. 1, the layout and construction of the conventional parking brake system requires the use of bulky spring actuators, a parking reservoir and associated pipework. All of these components require fitting and service which all adds to labour and material costs at the vehicle builders. Similar bulky spring actuators are usually also used in conventional pneumatic/mechanical (non-EBS) systems.
We have proposed in another Application filed concurrently hereto the provision of a parking braking system for a brake in a vehicle wherein, upon moving a parking brake selector to a park position, the brake is arranged to be mechanically locked or latched into place by means of a variable position park latch mechanism which is adapted to co-operate with an operating member of the brake so as to set the park load on the brake at a desired level.
In some embodiments, the variable position park latch mechanism can comprise a variable position backstop that is positioned so as to selectively abut the operating or input shaft of the brake.
The variable position backstop can be, for example, a cam or ratchet pin but is preferably a wedge, driven into position by a controlled actuator such as an air cylinder or preferably a motor.
An advantage of being able to park the brake at any prescribed level is the ability to accommodate changes in both the brake geometry and vehicle static condition. In particular, it is known that as a brake cools, during the time that a vehicle may be parked after a journey, the brake geometry changes, and can change in such a way that if a fixed park load condition was implemented, the clamp force being applied could reduce as the brake geometry relaxes. This would usually force the designer of a fixed park load system to over-clamp the brake to avoid the brake from being substantially released during cooling. This obviously induces greater stress and fatigue into the brake, both of which have to be accommodated in the brake design, thus producing a brake which is substantially over-engineered for the application. Furthermore, if it is possible to determine either or all of the vehicle static condition parameters such as operating gradient, axle load or brake temperature, then the park clamp load can be made to adapt to the prevailing vehicle condition.
A further advantage of a variable park latch system is that the build-up of basic brake condition tolerances, such as brake running clearance, new or worn linings and compressibility of the linings, again means that a single park load system, that clamps the brake at a fixed position, cannot take account of the variance in the aforementioned components without oversetting the clamp load.
Systems as described above can seek to overcome this problem by determining the level at which the park brake should be latched, driving the brake to a prescribed level using the foundation actuation system and then locking the brake at a desired level.
In preferred embodiments, a wedge is driven into engagement with a corresponding mating surface provided in the back of the operating shaft of the brake, by a motor driven threaded member. The threaded member assembly preferably comprises a non-reversible mechanism such as a high reduction ratio gearbox.
Particularly with embodiments using a cam or wedge, such systems have the ability to enable adjustment of the park load level during the parked condition through the use of the variable latch mechanism.
In accordance with the present invention there is provided a control system having a brake level determining means to determine the brake load level at which the brake is to be parked and a control means for controlling the actuation of the latching mechanism, or a combination of several latching mechanisms, to selectively maintain that brake load level during a parking phase.
Preferably, in order to determine the park load level to which the brake should be applied in a given situation, there is provided a static model which generates a signal representative of the static condition of the vehicle or a static compensation signal.
Preferably, the static model is arranged to derive the static compensation signal through assessment of the stationary condition of the vehicle. Conveniently, this is derived by way of example, from measurements taken from around the vehicle. A brake temperature signal can provide a signal indicative of the brake temperature at the time of park demand. A gradient sensor likewise can provide a signal that is proportional to the gradient upon which the vehicle is being parked, and an axle load sensor can provide information about the laden condition of the vehicle.
Advantageously, depending upon the level of complexity required, any one or more of the above signals may use used alone or in combination to provide the vehicle information required. Additionally, such signals may be derived by association, i.e. not by direct measurement. In particular, the brake temperature may be derived through assessment of the change in brake actuator travel during brake application, as brake travel for a given pressure can change with temperature of the brake.
Conveniently, a clamp load requirement can be produced for a given correlation between axle load and gradient. Only a low, say 10% of the maximum available, clamp load is required at both low gradients with high axle load as well as high gradients with a low axle load, subject to a minimum safe threshold. Likewise, a high, say 100% of the maximum available, clamp load is required for a brake on a vehicle carrying a high axle load on a high gradient. The output from this xe2x80x9cmapxe2x80x9d, preferably takes the form of a series of look-up tables in a non-volatile memory within the park latch controller.
Advantageously, the clamp load derived from the correlation between axle load and operating gradient may also be adjusted to take account of the relaxation with the brake as it cools. The brake temperature at the commencement of park can be used to introduce a proportional offset to the derived clamp load. The modified value deduced from this comparison can produce the static compensation signal that is used for setting the level to which the brake is actuated prior to the park latch being applied.
As mentioned hereinbefore, the present system is not limited to uses with EBS systems but is equally applicable to conventional brake actuation systems fitted with parking signalling to the park latch system.